Soluble, Degradable Poly(ethylene glycol) Derivatives for Controllable Release of Bound Molecules into Solution

ABSTRACT

PEG and related polymer derivatives having weak, hydrolytically unstable linkages near the reactive end of the polymer are provided for conjugation to drugs, including proteins, enzymes, small molecules, and others. These derivatives provide a sufficient circulation period for a drug-PEG conjugate, followed by hydrolytic breakdown of the conjugate and release of the bound molecule. In some cases, drugs that demonstrate reduced activity when permanently coupled to PEG maintain a therapeutically suitable activity when coupled to a degradable PEG in accordance with the invention. The PEG derivatives of the invention can be used to impart improved water solubility, increased size, a slower rate of kidney clearance, and reduced immunogenicity to a conjugate formed by attachment thereto. Controlled hydrolytic release of the bound molecule into an aqueous environment can then enhance the drug&#39;s delivery profile by providing a delivery system which employs such polymers and utilizes the teachings provided herein.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/414,621, filed Mar. 30, 2009, which is a continuation of U.S. patentapplication Ser. No. 11/051,728, filed Feb. 4, 2005, now U.S. Pat. No.7,655,747, which is a continuation of U.S. patent application Ser. No.10/318,322, filed Dec. 12, 2002, now U.S. Pat. No. 6,864,350, which is acontinuation of U.S. patent application Ser. No. 09/824,297, filed Apr.2, 2001, now U.S. Pat. No. 6,515,100, which is a divisional of U.S.patent application Ser. No. 08/937,846, filed Sep. 25, 1997, now U.S.Pat. No. 6,214,966, which claims the benefit of priority to ProvisionalPatent Application Ser. No. 60/026,716, filed Sep. 26, 1996, thecontents of which are all incorporated herein by reference in theirentireties.

FIELD OF THE INVENTION

This invention relates to active derivatives of poly(ethylene glycol)and related hydrophilic polymers having a reactive moiety at one end ofthe polymer chain suitable for chemical coupling to another molecule.

BACKGROUND OF THE INVENTION

Chemical attachment of the hydrophilic polymer poly(ethyleneglycol)(PEG), which is also known as poly(ethylene oxide) (PEO), tomolecules and surfaces is of great utility in biotechnology. In its mostcommon faun PEG is a linear polymer terminated at each end with ahydroxyl group:

HO—CH₂CH₂O—(CH₂CH₂O)_(n)—CH₂CH₂—OH

This polymer can be represented in brief form as “HO-PEG-OH” where it isunderstood that the -PEG- symbol represents the following structuralunit:

—CH₂CH₂O—(CH₂CH₂O)_(n)—CH₂CH₂—,

where n typically ranges from approximately 10 to approximately 2000.

PEG is commonly used as methoxy-PEG-OH, or “mPEG”, in which one terminusis the relatively inert methoxy group, while the other terminus is ahydroxyl group that is subject to ready chemical modification.

CH₃O—(CH₂CH₂O)_(n)—CH₂CH₂—OH mPEG

PEG is also commonly used in branched forms that can be prepared byaddition of ethylene oxide to various polyols, such as glycerol,pentaerythritol and sorbitol. For example, the four-arm, branched PEGprepared from pentaerythritol is shown below:

C(CH₂—OH)₄ +nC₂H₄O→C[CH₂O—(CH₂CH₂O)_(n)—CH₂CH₂—OH]₄

Such branched polyethylene glycols can be represented in general form asR(-PEG-OH)_(z) in which R represents the central “core” molecule, suchas glycerol or pentaerythritol, and z represents the number of armsextending therefrom.

PEG is a well-known polymer having the following properties: solubilityin water and in many organic solvents, lack of toxicity, and lack ofimmunogenicity. One use of PEG is to covalently attach the polymer toinsoluble molecules to make the resulting PEG-molecule conjugatesoluble. For example, Greenwald, Pendri and Bolikal in J. Org. Chem.,60, 331-336 (1995), have shown that the water-insoluble drug, taxol,when coupled to PEG, becomes water-soluble.

In related work, Davis et al., in U.S. Pat. No. 4,179,337, have shownthat proteins coupled to PEG have enhanced blood-circulation lifetimesdue to reduced rate of kidney clearance and reduced immunogenicity.Hydrophobic proteins have been described that, upon coupling to PEG,have increased solubility in water. These and other uses for PEG aredescribed in J. M. Harris, Ed., “Biomedical and BiotechnicalApplications of Polyethylene Glycol Chemistry,” Plenum, New York, 1992).

To couple PEG to a molecule such as a protein or onto a surface, it isnecessary to use an “activated derivative” of the PEG having afunctional group at a terminus suitable for reacting with some group onthe protein or on the surface (such as an amino group). Among the manyuseful activated derivatives of PEG is the succinimidyl “active ester”of carboxymethylated PEG as disclosed by K. Iwasaki and Y. Iwashita inU.S. Pat. No. 4,670,417. This chemistry can be illustrated with theactive ester reacting with amino groups of a protein (the succinimidylgroup is represented as “NHS” and the amino-containing protein isrepresented as PRO—NH₂):

PEG-O—CH₂—CO₂—NHS+PRO—NH₂→PEG-O—CH₂—CO₂—NH—PRO

Certain problems have arisen in the art. Some of the functional groupsthat have been used to activate PEG can result in toxic or otherwiseundesirable residues when used for in vivo drug delivery. Some of thelinkages that have been devised to attach functional groups to PEG canresult in an undesirable immune response. Additionally, certainfunctional groups do not have appropriate selectivity for reacting withparticular groups on proteins and can deactivate the proteins when inconjugated form.

Attachment of a PEG derivative to a substance can have a somewhatunpredictable impact on the substance. Proteins, small drugs, and thelike may have reduced biological activity when conjugated to a PEGderivative, although in some cases, activity may be increased.

Another example of a problem that has arisen in the art is exemplifiedby the succinimidyl succinate active ester, “mPEG-SS” (the succinimidylgroup is represented as NHS):

The mPEG-SS active ester is useful for coupling because it reactsrapidly with amino groups on proteins and other molecules to form anamide linkage (—CO—NH—). A problem has been reported with the mPEG-SSactive ester, as noted in U.S. Pat. No. 4,670,417, Since this compoundpossesses an ester linkage in the backbone that remains after couplingto an amine group, such as in a protein (represented as PRO—NH₂):

mPEG-SS+PRO—NH2→mPEG-OC(O)—CH₂CH₂—CONH—PRO,

the remaining ester linkage is subject to rapid hydrolysis anddetachment of PEG from the modified protein. Too rapid a hydrolysis ratecan preclude use of a PEG derivative for many applications. Severalapproaches have been adopted to solve the problem of hydrolyticinstability. For example, mPEG succinimidyl carbonate has been proposed,which contains only ether linkages in the polymer backbone and reactswith proteins to form a conjugate that is not subject to hydrolysis.

In view of the above, it would be desirable to provide alternative PEGderivatives that are suitable for drug delivery systems, includingdelivery of proteins, enzymes, and small molecules, or for otherbiotechnology uses. It would also be desirable to provide alternativePEG derivatives that could enhance drug delivery systems orbiotechnology products.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides chemically activepolyethylene glycols and related polymers that are suitable for couplingto other molecules. The polyethylene glycol derivatives provided herein,when covalently attached to another molecule, form water-solubleconjugates in which the linkage between the polymer and the boundmolecule is subject to predetermined cleavage. This allows forcontrolled delivery of the bound molecule into its surroundingenvironment.

The polymers of the invention contain weak, hydrolytically unstablelinkages near the reactive end of the polymer that provide for asufficient circulation period for a drug-PEG conjugate, followed byhydrolytic breakdown of the conjugate and release of the bound molecule.

According to yet another aspect, also forming part of the presentinvention are (i) methods of preparing the degradable active PEGsdescribed herein and related polymers, (ii) conjugates, and (iii)methods for preparing such PEG conjugates.

The PEG and related polymer derivatives of the invention are capable ofimparting improved water solubility, increased size, a slower rate ofkidney clearance, and reduced immunogenicity to a conjugate formed bycovalent attachment thereto, while also providing for controllablehydrolytic release of the bound molecule into an aqueous environment—byvirtue of the design of the linkages provided herein. The invention canbe used to enhance the solubility and blood circulation lifetime ofdrugs in the bloodstream, while also delivering a drug into thebloodstream that, subsequent to hydrolysis, is substantially free ofPEG. The invention is especially useful in those cases where drugs, whenpermanently conjugated to PEG, demonstrate reduced activity. By using adegradable PEG as provided herein, such drugs can maintain theirtherapeutic activity when in conjugated form, by virtue of the cleavablenature of the PEGs of the invention.

In general form, the polymer derivatives of the invention and theirconjugates can be described by the following equations:

POLY(-W-T)_(y)+Y-P′→POLY(-W-X-P′)_(y)+H₂O→

→POLY-G_(y)+(I-X-P′)

In the above equations,

“POLY” is a linear or branched polyethylene glycol of molecular weightfrom 300 to 100,000 daltons. POLY can also be a related non-peptidicpolymer as described in greater detail below;

“y” is the number of chemically active groups on POLY and is the numberof molecules that can be bound to POLY;

W is a hydrolytically unstable group;

T is a reactive group (i.e., is reactive with Y);

Y-P′ represents a molecule for conjugation to POLY, whereY is a reactivegroup that is reactive with T, and P′ is the portion of the moleculethat is to be bound and released, including, for example, a peptide P′in which Y is an amine moiety and T is a PEG activating moiety reactivewith an amine;

X is the new linkage formed by reaction of Y and T; and

G and I are new groups formed by hydrolysis of W.

Examples of hydrolytically unstable groups, W, include carboxylateesters, phosphate esters, acetals, imines, orthoesters, peptides andoligonucleotides. The variables, T and Y, are groups that react witheach other. There are many examples of such groups known in organicchemistry. Some examples include the following combinations: activeesters reacting with amines, isocyanates reacting with alcohols andamines, aldehydes reacting with amines, epoxides reacting with amines,and sulfonate esters reacting with amines, among others.

Examples of P′ include peptides, oligonucleotides and otherpharmaceuticals.

Examples of X, the linkage resulting from reaction of Y and T, includeamide from reaction of active esters with amine, urethane from reactionof isocyanate with hydroxyl, and urea from reaction of amine withisocyanate.

Examples of G and I, the new groups formed upon hydrolysis of W, arealcohol and acid from hydrolysis of carboxylate esters, aldehyde andalcohol from hydrolysis of acetals, aldehyde and amine from hydrolysisof imines, phosphate and alcohol from hydrolysis of phosphate esters,amine and acid from hydrolysis of peptide, and phosphate and alcoholfrom hydrolysis of oligonucleotides.

An embodiment of the invention is shown in the following equationdemonstrating conjugation of a hydrolyzable methoxy-PEG (mPEG) polymerderivative with a peptide drug, followed by hydrolytic release of thepeptide. In the embodiment below, the weak linkage, W, contains ahydrolyzable ester group.

The released exemplary peptide contains no polymer, in this case, mPEG.Rather, the released peptide contains an additional short molecularfragment, which is sometimes called a “tag”. This tag is the portion ofthe linkage opposite the PEG from the hydrolytically unstable linkage.In the above example, the tag portion is the ‘HO—(CH₂)₃—O—C(O)—’ whichremains attached to the released peptide.

Thus, the invention provides activated PEGs and other related polymerscontaining hydrolytically unstable linkages. The polymers are useful,when conjugated to a drug or other molecule, for controlled delivery ofsuch molecule to its surrounding environment. Several types of linkages,including ester linkages, are suitable for use as the hydrolyticallyunstable linkage as provided herein. However, the ester linkages of thepresent polymers, in contrast to mPEG-SS and mPEG-SG, provide forvariation and control of the rate of hydrolytic degradation.

The foregoing and other objects, advantages, and features of theinvention, and the manner in which the same are accomplished, will bemore readily apparent upon consideration of the following detaileddescription of the invention taken in conjunction with the accompanyingdrawings, which illustrate an exemplary embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a MALDI-MS spectrum of the molecular weight distribution of anmPEG-HBA- subtilisin conjugate at 1 day after preparation as describedin Example 8.

FIG. 2 is a MALDI-MS spectrum of the molecular weight distribution of anmPEG-HBA- subtilisin conjugate at 8 days after preparation as describedin Example 8.

FIG. 3 is a MALDI-MS spectrum of the molecular weight distribution of anmPEG-HBA- subtilisin conjugate at 14 days after preparation as describedin Example 8.

DETAILED DESCRIPTION

The following detailed description describes various embodiments of theinvention as described by the following general equations presented inthe preceding summary:

Poly(-W-T)_(y)+Y-P′→Poly(-W-X-P′)_(y)+H₂O→

→Poly-G_(y)+I-X-P′

The Polymer

In the discussion below, POLY will often be referred to for convenienceas PEG or as poly(ethylene glycol). However, it is to be understood thatother related polymers are also suitable for use in the practice of theinvention in place of PEG and that the use of the term PEG orpoly(ethylene glycol) is intended to be inclusive and not exclusive inthis respect.

Poly(ethylene glycol) is preferred in the practice of the invention. PEGis used in biological applications because it has properties that arehighly desirable and is generally approved for biological orbiotechnological applications. PEG typically is clear, colorless,odorless, soluble in water, stable to heat, inert to many chemicalagents, does not hydrolyze or deteriorate, and is non-toxic.Poly(ethylene glycol) is considered to be biocompatible, which is to saythat PEG is capable of coexistence with living tissues or organismswithout causing harm. More specifically, PEG is not immunogenic, whichis to say that PEG does not tend to produce an immune response in thebody. When attached to a moiety having some desirable function in thebody, the PEG tends to mask the moiety and can reduce or eliminate animmune response so that an organism can tolerate the presence of themoiety, in PEGylated form. Accordingly, the activated PEGs of theinvention should be substantially non-toxic and not tend tosubstantially produce an immune response or cause clotting or otherundesirable effects.

Water-soluble polymers other than PEG are suitable for similarmodification in accordance with the invention described herein. Theseother polymers include poly(vinyl alcohol) (“PVA”); other poly(alkyleneoxides) such as poly(propylene glycol) (“PPG”) and the like; andpoly(oxyethylated polyols) such as poly(oxyethylated glycerol),poly(oxyethylated sorbitol), and poly(oxyethylated glucose), and thelike. The polymers can be homopolymers or random or block copolymers andterpolymers based on monomers of the above polymers, straight chain orbranched, substituted or unsubstituted, e.g., similar to mPEG and othercapped, mono-functional PEGs having a single active site available forattachment to a linker.

Specific examples of such suitable polymers include poly(oxazoline),poly(acryloylmorpholine) (“PAcM”), and poly(vinylpyrrolidone)(“PVP”).PVP and poly(oxazoline) are well-known polymers in the art and theirpreparation and use in the syntheses described herein for embodimentsbased upon mPEG should be readily apparent to the skilled artisan. PAcMand its synthesis and use are described in U.S. Pat. Nos. 5,629,384 and5,631,322, the contents of which are incorporated herein by reference intheir entireties.

DEFINITIONS

It should be understood that by “drug” is meant any substance intendedfor the diagnosis, cure, mitigation, treatment, or prevention of diseasein humans and other animals, or used to otherwise enhance physical ormental well-being. The invention is useful, for example, for delivery ofbiologically active substances that generally have some activity orfunction in a living organism or in a substance taken from a livingorganism.

The terms “group,” “functional group,” “moiety,” “active moiety,”“reactive site,” and “radical” are all somewhat synonymous in thechemical arts and are used in the art and herein to refer to distinct,definable portions or units of a molecule and to units that perform somefunction or activity and are reactive with other molecules or portionsof molecules.

The term “linkage” is used herein to refer to groups that are normallyformed as the result of a chemical reaction. Herein, such linkages aretypically covalent linkages.

“Hydrolytically stable linkage” means a linkage that is stable in waterand does not react with water under useful or normal conditions of pHfor an extended period of time, potentially indefinitely.

“Hydrolytically unstable linkage” is a linkage or functionality thatreacts with water, typically causing a molecule to separate (or cleave)into two or more components. Such a linkage is said to be “subject tohydrolysis” and to be “hydrolysable”. The time it takes for the linkageto react with water is referred to as the rate of hydrolysis and isusually measured in terms of its half-life.

The Hydrolytically Unstable Linkage (W) and Reactive Groups, T and Y

In a particular embodiment, the invention includes poly(ethyleneglycols) containing, for example, an ester group as the weak linkage anda succinimidyl ester as the reactive group useful for coupling toamine-containing molecules. The resulting conjugates can be delivered invivo or into a substance taken from a living entity. Exemplary polymers,and in particular, linkages, are provided below.

PEG-W—CO₂—NHS

where W=

-   -   O₂C—(CH₂)_(b)—O— b=1-5    -   —O—(CH₂)_(b)CO₂—(CH₂)_(c)— b=1-5, c=2-5    -   —O—(CH₂)_(b)—CO₂—(CH₂)_(n)—O— b=1-5, c=2-5

The invention also encompasses poly(ethylene glycols) containing anester group as the weak linkage and an isocyanate as the reactive groupuseful for coupling to amine- and alcohol-containing molecules, asexemplified below.

PEG-W—N═C═O

where W=O—(CH₂)_(b)—CO₂—(CH₂)_(c)-b=1 to 5, c=2 to 5

As a further example, the invention also includes poly(ethylene glycols)containing an acetal as the weak linkage and a succinimidyl ester as thereactive group useful, for example, for coupling to an amine-containingmolecule.

PEG-W—CO₂—NHS,

where, for example, W equals:

In the above illustrative structures,o=1-10,Z=—O—C₆H₄— and —O—(CH₂)_(d)—CH₂— d=1-5R′=alkyl or H.

In yet another embodiment, the invention encompasses poly(ethyleneglycols) containing an imine group as the weak linkage and asuccinimidyl ester as the reactive group, e.g., useful for coupling to,for example, an amine-containing molecule.

PEG-W—CO₂—NHS

where

-   -   W=—Z—CH═N—(CH₂)_(b)—O— b=1-5    -   Z=—O—C₆H₄        -   —O—(CH₂)_(b)—CH₂— b=1-5

According to yet another embodiment, the invention also includespoly(ethylene glycols) containing a phosphate ester group as the weaklinkage and a succinimidyl ester as the reactive group, e.g., useful forcoupling to an amine-containing molecule.

PEG-W—CO₂—NHS

where W=—(CH₂)_(b)—OPO₃—(CH₂)_(b′)— b and b′=1-5

According to yet another exemplary embodiment, the invention includespoly(ethylene glycols) containing an ester-linked amino acid as the weaklinkage and a succinimidyl ester as the reactive group, e.g., useful forcoupling to an amine-containing molecule. An advantage of thisderivative is that its hydrolytic breakdown results in a biologicallyacceptable amino acid attached to the released molecule:

PEG-W—CO₂—NHS

where W=O—(CH₂)_(b)—CO₂—(CH₂)_(b′)—CH(NH-t-Boc)-

-   -   b=1-5,    -   b′=1-5, and    -   t-Boc=(CH₃)₃C—O—CO—.

In yet a further embodiment, the invention includes poly(ethyleneglycols) containing a peptide as the weak linkage and a succinimidylester as the reactive group useful for coupling to an amine-containingmolecule. An advantage of this particular type of derivative is that itshydrolytic breakdown typically results in a biologically acceptablepeptide fragment attached to the released molecule:

PEG-W—CO₂—NHS

where W=

-   -   —C(O)(NH—CHR—CO)_(a)—NH—CHR— a=2-20    -   R=the set of substituents typically found on α-amino acids

In yet another exemplary embodiment, the invention includespoly(ethylene glycols) containing an oligonucleotide forming the weaklinkage and a succinimidyl ester as the reactive group, e.g., useful forcoupling to an amine-containing molecule. An advantage of thisparticular type of derivative is that its hydrolytic breakdown resultsin a biologically acceptable oligonucleotide fragment attached to thereleased molecule:

PEG-W—CO₂—NHS

where W=oligonucleotide.

As previously described, polymers for use in the invention can bestraight chain or branched. That is to say, branched activated PEGs canbe prepared in accordance with the invention where such PEGs possessweak linkages near the reactive end of the polymer for controlledhydrolytic degradation. Illustrative branched PEGs are described inInternational Publication No. WO 96/21469, entitled, “Multi-Armed,Monofunctional, and Hydrolytically Stable Derivatives of Poly(EthyleneGlycol) and Related Polymers For Modification of Surfaces andMolecules”, filed Jan. 11, 1996, the content of which is incorporatedherein by reference in its entirety. Branched PEGs such as these canthen be modified in accordance with the present teachings.

The invention is illustrated with respect to several particular examplesbelow, including determination of hydrolysis half-lives forrepresentative hydrolyzable polymer derivatives and conjugates.

EXAMPLES Example 1 PREPARATION OF: CH₃O-PEG-O—CH₂—COO—CH₂—COOH,“mPEG-CM-GA-NHS” CH₃O-PEG-O—(CH₂)₂—COO—CH₂—COOH, “mPEG-PA-GA-NHS”

Reactions (n=1 or 2):

CH₃O-PEG₃₀₀₀-O—(CH₂)_(1,2)—COOH (3.0 g, 1 mmol, mPEG-CM or mPEG-PA) wasazeotropically dried with 60 ml of toluene under N₂. After two hours,the solution was cooled to room temperature, followed by injection of asolution of thionyl chloride (2 ml, 4 mmol) in CH₂Cl₂. The resultingsolution was stirred at room temperature overnight. The solvent wascondensed on a rotary evaporator and the residual syrup was dried invacuo for about four hours over P₂O₅ powder.

Glycolic acid (0.2 g, 2.7 mmole) was azeotropically distilled with 70 mlof 1,4-dioxane and the distillation was stopped when approximately 20 mlof solution remained. The solution was slowly cooled to room temperatureunder N₂. The glycolic acid/dioxane solution was then added to the driedPEG acyl chloride. After the PEG was dissolved, 0.6 ml of drytriethylamine was injected to the system (precipitate formedimmediately) and the solution was stirred overnight. The salt wasremoved by filtration and the filtrate was condensed on a rotaryevaporator at 55° C. and dried in vacuo. The crude product was thendissolved in 100 ml of distilled water and the pH of the solution wasadjusted to 3.0. The aqueous phase was extracted three times with atotal of 80 ml of methylene chloride. The combined organic phase wasdried over sodium sulfate, filtered to remove salt, condensed on arotary evaporator, and added to 100 ml of ethyl ether. The precipitatewas collected by filtration and dried in vacuo.

Yield 2.55 g (85%). ¹H NMR (DMSO-d₆): δ 3.5 (br m, PEG), 4.3-4.6 (s,PEGOCH₂COOCH2COOH), 2.59 (t, PEGOCH₂CH ₂COO (PA)), 4.19 (s, PEGOCH ₂COO(CM)).

Example 2 PREPARATION OF HOOC—CH₂—OOC—CH₂—O-PEG-O—CH₂—COO—CH₂—COONReactions:

Difunctional carboxymethyl PEG_(20,000)-ester benzyl glycolate:Difunctional carboxymethyl PEG 20,000 (4 gram, 0.4 mmol acid group),benzyl glycolate (0.6 mmol), dimethylaminopyridine (0.44 mmol),1-hydroxybenzotriazole (0.4 mmol) and dicyclohexylcarbodiimide (0.56mmol) were dissolved in 40 ml of methylene chloride. The solution wasstirred at room temperature under N₂ overnight. The solvent was thenremoved under vacuum and the resulting residue was added to 20 ml oftoluene at 40° C. The undissolved solid was removed by filtration andthe filtrate was added to 200 ml of ethyl ether. The precipitate wascollected by filtration and dried in vacuo.

Yield: 4 gram (100%). ¹H NMR (DMSO-d₆): δ 3.5 (br m, PEG), 4.81 (s,PEGOCH ₂COOCH₂COOCH₂C₆H₅), 5.18 (s, PEGOCH₂COOCH₂COOCH ₂C₆H₅), 7.37 (s,PEGOCH₂COOCH₂COOCH₂C₆H₅), 4.24 (s, PEGOCH₂COOCH₂COOCH₂C₆H₅).

Difunctional carboxymethyl PEG-ester benzyl glycolate 20,000 (3 gram)and Pd/C (10%, 0.8 gram) were added to 30 ml of 1,4-dioxane. The mixturewas shaken with H₂ (40 psi) at room temperature overnight. The Pd/C wasremoved by filtration and the solvent was condensed by rotaryevaporation. The resulting syrup was added to 100 ml of ether. Theprecipitated product was collected by filtration and dried in vacuo.

Yield 2.4 gram (80%). ¹H NMR (DMSO-d₆): δ 3.5 (br m, PEG), 4.56 (s,PEGOCH₂COOCH ₂COOH), 4.20 (s, PEGOCH ₂COOCH₂COOH).

Example 3 PREPARATION OF CH₃O-PEG-O—(CH₂)_(1,2)—COO—CH₂—COONHS

Reactions (n=1 or 2):

CH₃O-PEG-O—(CH₂)_(1,2)—COO—CH₂—COOH (1 g, approx. 0.33 mmol) and 42 mgN-hydroxysuccinimide (NHS) (0.35 mmol) was dissolved in 30 ml of drymethylene chloride. To this was added dicyclohexylcarbodiimide (DCC) (80mg, 0.38 mmol) in 5 ml of dry methylene chloride. The solution wasstirred under nitrogen overnight and the solvent was removed by rotaryevaporation. The resulting syrup was re-dissolved in 10 ml of drytoluene and the insoluble solid was filtered off. The solution was thenprecipitated into 100 ml of dry ethyl ether. The precipitate wascollected by filtration and dried in vacuo.

Yield 0.95 g (95%). ¹H NMR (DMSO-d₆): δ 3.5 (br m, PEG), 5.15-5.21 (s,PEGOCH₂COOCH ₂COONHS), 2.67 (t, PEGOCH₂CH ₂COO (PA)), 4.27 (s, PEGOCH₂COO ppm(CM)), 2.82 (s, NHS, 4H).

Example 4 Preparation of

Reactions (n=1 or 2):

CH₃O-PEG-O—(CH₂)_(1,2)—COO—CH₂—COOH (1.5 g, approx 0.5 mmol), 140 mgp-nitrophenol (1 mmol) and 65 mg dimethylaminopyridine (DMAP) (0.525mmol) were dissolved in 30 ml of dry methylene chloride. To theresulting solution was added dicyclohexylcarbodiimide (DCC) (120 mg,0.575 mmole) in 5 ml of dry methylene chloride. The solution was stirredunder nitrogen overnight and the solvent was removed by rotaryevaporation. The resulting syrup was redissolved in 10 ml of dry tolueneand the insoluble solid was removed by filtration. Then the solution wasprecipitated into 100 ml of dry ethyl ether. The product wasreprecipitated with ethyl ether, then collected by filtration and driedin vacuo.

Yield 1.425 g (95%). ¹H NMR (DMSO-d₆): δ 3.5 (br m, PEG), 5.01 (s,PEGOCH₂COOCH ₂COONP), 2.69 (t, PEGOCH ₂CH₂COO (PA)), 8.35 & 7.48 (d&d,H_(a) & H_(b) in NP, 4H).

Example 5 PREPARATION OF CH₃O-PEG-O—(CH₂)_(n)—COO—CH(CH₃)CH₂—COONHS(n=1: MPEG-CM-HBA-NHS AND n=2: MPEG-PA-HBA-NHS

Reactions (n=1 or 2):

CH₃O-PEG-O—(CH₂)_(n)—COOH 3000 (3.0 g, 1 mmol) was azeotropically driedwith 60 ml of toluene under N₂. After two hours, the solution was slowlycooled to room temperature. To the resulting solution was added thionylchloride solution (3 ml, 6 mmol) in CH₂Cl₂, and the solution was stirredovernight. The solvent was condensed by rotary evaporation and the syrupwas dried in vacuo for about four hours over P₂O₅ powder.3-hydroxybutyric acid (HBA, 0.30 g, 2.7 mmol) was azeotropically driedwith 70 ml of 1,4-dioxane on a rotary evaporator. The distillation wasstopped when approximately 20 ml of solution remained. This solution wasthen slowly cooled to room temperature under N₂, and the solution wasadded to the dried PEG acyl chloride. After the PEG was dissolved, 0.6ml of dry triethylamine was injected to the system (precipitate formedimmediately) and the solution was stirred overnight. The salt wasremoved by filtration and the filtrate was condensed on a rotaryevaporator at 55° C. and dried in vacuo. The crude product was thendissolved in 100 ml of distilled water and the pH of the solution wasadjusted to 3.0. The aqueous phase was extracted three times with atotal of 80 ml of methylene chloride. The organic phase was dried oversodium sulfate, filtered to remove salt, condensed on a rotaryevaporator, and added to 100 ml of ethyl ether. The precipitate wascollected by filtration and dried in vacuo. Yield 2.76 g (92%). ¹H NMR(DMSO-d₆): δ 3.5 (br m, PEG), 2.54 (d, PEGOCH₂COOCH(CH₃)CH ₂COOH), 5.1(h, PEGOCH₂COOCH(CH₃)CH₂COOH), 1.2 (d, PEG-OCH₂COOCH(CH ₃)CH₂COOH), 2.54(t, PEGOCH₂CH ₂COO (PA)), 4.055 (s, PEGOCH ₂COO (CM)).

mPEG-ester butyric acid NHS ester (CM-HBA-NHS or PA-HBA-NHS):mPEG₃₀₀₀-ester butyric acid (1g, approx., 0.33 mmol, CM-HBA-COOH orPA-HBA-COOH) and 42 mg N-hydroxysuccinimide (NHS) (0.35 mmol) weredissolved in 30 ml of dry methylene chloride. To this solution was addeddicyclohexylcarbodiimide (DCC) (80 mg, 0.38 mmole) in 5 ml of drymethylene chloride. The solution was stirred under nitrogen overnightand the solvent removed by rotary evaporation. The residual syrup wasre-dissolved in 10 ml of dry toluene and the insoluble solid was removedby filtration. The solution was then precipitated into 100 ml of dryethyl ether. The precipitate was collected by filtration and dried invacuo.

Yield 0.94 g (94%). ¹H NMR (DMSO-d₆): δ 3.5 (br m PEG), 3.0-3.2 (m,COOCH(CH₃)CH ₂COONHS), 5.26 (h, COOCH(CH₃)CH₂—COONHS), 1.3 (d, COOCH(CH₃)CH₂COONHS), 2.54 (t, OCH₂CH ₂COO (PA)), 4.1 (s, OCH ₂COO (CM)), 2.81(s, NHS).

Example 6 Determination of the Hydrolytic Half-Lives of the EsterLinkages Contained in Four Exemplary PEG-PEG Conjugates

Reactions (n=1 or 2):

A. Preparation of CH₃O-PEG-O—(CH₂)_(n)—COO—CO₂—CONH-PEG-OCH₃ (PEG-PEGconjugates): CH₃O-PEG₃₀₀₀-O—(CH₂)_(n)—COO—CH₂—COOH (0.5 g), 1 equiv. ofmPEG₂₀₀₀-NH₂ and 1 equiv. of 1-hydroxybenzotriazole (HOBT) weredissolved in 50 ml of methylene chloride. To this solution was added oneequivalent of dicyclohexylcarbodiimide (DCC) and the solution wasstirred at room temperature overnight. The solvent was partiallyevaporated, the insoluble salt was filtered, and the filtrate was addedinto a large excess of ethyl ether. The precipitate was collected byfiltration and dried in vacuo. Yield: 0.8 g (95%). ¹H NMR (DMSO-d₆): δ3.5 (br m, PEG), 2.34 (t, —CONHCH ₂CH₂O-PEG-).

B. Determination of hydrolytic half-lives of PEG ester conjugates formedby reaction of CM-GA, PA-GA, CM-HBA or PA-HBA with a PEG amine: Theconjugates from A. above and 20 wt % PEG 20,000 (as an internalstandard) were dissolved in a buffer solution. The concentrations ofeach of the conjugates (C) and their hydrolysis products were monitoredby HPLC-GPC (Ultrahydrogel 250 column, 7.8×300 mm, Waters) atpredeteimined times. The hydrolytic half-lives were obtained from theslope of the natural logarithm of C at the time, t, minus C at infinitetime versus time assuming 1^(st) order kinetics.

TABLE 1 HYDROLYSIS HALF-LIVES (DAYS, UNLESS NOTED OTHERWISE) OF THEESTER LINKAGES CONTAINED IN THE ABOVE CONJUGATES. (±10%) Double-EsterPEG Used to Form Conjugate CM-GA PA-GA CM-HBA PA-HBA pH 7.0 7.0 8.1 7.08.1 7.0 8.1 23° C. 3.2 43 6.5 — 15 — 120 37° C. 14 h 7.6 — 14 — 112 —50° C.  4 h 2.2 — 5 — 58 —

Example 7 Determination of Hydrolysis Half-Lives of the Ester LinkagesContained in Four Exemplary PEG Reagents

Reactions (n=1 or 2):

Determination of hydrolysis half-lives of PEG active esters:Measurements were conducted using a HP8452a UV-VIS spectrophotometer. Inan experiment, 1 mg of a given PEG active ester was dissolved in 3.0 mlof buffer solution and shaken promptly to obtain dissolution as soon aspossible. The solution was then transferred into a UV cuvette and theabsorbance at 260 nm (for NHS ester) or at 402 nm (for the p-nitrophenylester) was followed as a function of time. The hydrolytic half-life wasdetermined from the first order kinetic plot (natural logarithm of finalabsorbance minus absorbance at the time t versus time).

TABLE 2 HYDROLYSIS HALF-LIVES OF SUCCINIMIDYL ACTIVE ESTERS (R = NHS)AND P-NITROPHENYL ACTIVE ESTERS (R = NP) OF PEG-ESTER ACIDS AT PH 8.1AND ROOM TEMPERATURE R CM-GA-R PA-GA-R CM-HBA-R PA-HBA-R NHS 11 s 11 s12 min 12 min NP  7 min  7 min — —

Example 8 Hydrolytic Release of Peg from a PEG-Protein Conjugate byMALDI-TOF Mass Spectrometry

Modification of subtilisin with an illustrative PEG derivative: To asubtilisin solution (1 ml, 2 mg/ml in 0.2M boric buffer, pH 8.0) wasadded 15 mg mPEG₃₀₀₀-CM-HBA-NHS. The solution was placed in an automaticshaker at room temperature. At predetermined time periods, 50 μl of thesolution was removed and preserved in a refrigerator for MALDI-TOF MSmeasurement.

MALDI Analyses: MALDI spectra were measured on a PerSeptive Biosystems'Voyager linear time-of-flight (TOF) instrument. Briefly, a nitrogenlaser lamda=337 nm, 10 ns pulse width) was used to generate ions whichwere extracted with a potential of 30 kV. Ions drifted through a 1.3 mdrift tube and were monitored in positive ion mode.

Protein samples were dissolved in deionized H₂O or 50 mM NaCl solutionto a concentration of approximately 10 pmol/μl. The matrix,3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid), was dissolved inan 80:20 by volume ratio of acetonitrile to deionized water at aconcentration of 10 mg/ml. 1 μl of the solution was deposited on thesample plate and then mixed with 1 μl of matrix solution. The sample wasallowed to crystallize by solvent evaporation under ambient conditions.

MALDI-MS spectra of the molecular weight distribution of mPEG-HBA andits subtilisin conjugate are shown in FIGS. 1 through 3. Each of thefigures represents the solution at a different time subsequent topreparation. FIG. 1 is at 1 day. FIG. 2 is at 8 days. FIG. 3 is at 14days.

Example 9 Preparation of

Reactions:

CH₃O-PEG-O—CH₂—COOH 5000 (3.0 g, 0.6 mmol), 2-(2-pyridyldithio)ethanol(342 mg, 1.5 mmol), DMAP (180 mg, 1.44 mmol) and HOBT (93 mg, 0.61 mmol)were dissolved in 60 ml of dichloromethane. To this solution was addedDCC (138 mg, 0.66 mmol) in 5 ml of dichloromethane. The solution wasstirred at room temperature under N₂ overnight. The solvent was removedby rotary evaporation and 15 ml of toluene was added to the residue.After all PEG was dissolved, the solution was filtered to removedicyclohexyl urea. To the solution was added 45 ml of methylene chlorideand the solution was washed with sodium acetate buffer (0.1M, pH 5.0)containing 10% sodium chloride. The organic phase was dried overanhydrous sodium sulfate, filtered to remove salt, condensed on a rotaryevaporator, and precipitated into 100 ml of ethyl ether. The product wascollected by filtration and dried in vacuo. Yield 2.85 g (95%). ¹H NMR(DMSO-d₆): δ 3.5 (br m, PEG), 4.11 (s, PEGOCH ₂COO—), 4.30 (t, COOCH₂CH₂SS—) 7.29 (t, one aromatic proton), 7.77 (t+d, two aromaticprotons), 8.46 (d, one aromatic proton).

Example 10 Determination of the Hydrolysis Half-Life of an Ester Linkagein an Illustrative PEG Derivative Reactions:

mPEG-CM-SSP and 20% PEG 20,000 (wt) (as internal standard) weredissolved in 10 mM phosphate buffer (pH 7.2) and a series of ampouleswere sealed with each containing about 0.25 ml of above solution. Theampoules were stored as two groups, with one group at room temperatureand the other at 37° C. For each measurement, one ampoule in each groupwas opened and the solution was analyzed. The concentrations ofmPEG-CM-SSP and its hydrolysis product were determined by HPLC-GPC(Ultrahydrogel 250 column, Waters; 5 mM phosphate buffer pH 7.2 asmobile phase). The hydrolytic half-life was obtained from the slope ofthe natural logarithm of C at the time t minus C at infinite time versustime, assuming 1st order kinetics.

TABLE 3 HYDROLYTIC HALF-LIVES (DAYS) OF THE ESTER IN MPEG-CM-SSP. (10%)pH 5.5 pH 7.0 Room temperature 107 18 37° C. 20 2.9

Example 11 PREPARATION OF CH₂O-PEG-O(CH₂)_(n)—CO₂-PEG-OCOONHS

Reactions (n=1 or 2):

(a) Preparation of CH₃O-PEG-OCH₂CH₂CO₂-PEG-OBz

In a 100 ml round-bottom flask, a solution of CH₃O-PEG-O—(CH₂)_(n)—CO₂H(MW=2000, 2 g, 1 mmol) was dissolved in toluene and azeotropically driedfor two hours. After slowly cooling to room temperature, the solutionwas added to thionyl chloride (3 ml, 6 mmole) in methylene chloride andthen stirred under N₂ overnight. The solvent was then removed by rotaryevaporation and the residual syrup was dried in vacuo for about fourhours over P₂O₅ powder. To the solid was added 5 ml of anhydrousmethylene chloride and a solution (20 ml) of azeotropically driedBzO-PEG-OH (MW=3400, 2.04 g, 0.60 mmol) in toluene. To the resultingsolution was added 0.6 ml of freshly distilled triethylamine and thesolution was stirred overnight. The triethylamine salt was removed byfiltration and the crude product was precipitated with ethyl ether andcollected by filtration. The mixture was then purified by ion-exchangechromatography (DEAE sepharose fast flow column, Pharmacia). PureCH₃O-PEG-O—(CH₂)_(n)—CO₂-PEG-OBz was obtained. Yield: 2.6 g (80%). NMR(DMSO-d₆): δ 3.5 (br m, PEG), 2.55 (t, —OCH₂CH ₂COOPEG-), 4.14 (s,-PEGOCH ₂COOPEG-), 4.13 (t, -PEGOCH₂CH₂—COOCH ₂CH₂OPEG-), 4.18 (t,-PEGOCH₂—COOCH ₂CH₂OPEG), 4.49 (s, -PEG-O—CH ₂—C₆H₅), 7.33 (s+com,-PEG-O—CH₂—C₆ H ₅).

(b) Preparation of CH₃O-PEG-O—(CH₂)_(n)—CO₂-PEG-OH

A solution of 2 g of CH₃O-PEG-O—(CH₂)_(n)—CO₂-PEG-OBz in 1,4-dioxane washydrogenolyzed with H₂ (2 atm) on 1 gram Pd/C (10%) overnight. Thecatalyst was removed by filtration, the solvent was condensed undervacuum and the solution was added to ethyl ether. The product wascollected by filtration and dried under vacuum at room temperature toyield: 1.5 g (75%) of CH₃O-PEG-O—(CH₂)_(n)—CO₂-PEG-OH. NMR (DMSO-d₆): δ3.5 (br m, PEG), 2.55 (t, —OCH₂CH ₂COOPEG-), 4.14 (s, -PEG-OCH₂COOPEG-), 4.13(t, —PEGOCH₂CH₂COOCH ₂CH₂OPEG-), 4.18 (t, -PEGOCH₂—COOCH₂CH₂OPEG).

(c) Preparation of CH₃O-PEG-O—(CH₂)_(n)—COO₂-PEG-OCOONHS

CH₃O-PEG-O—(CH₂)_(n)-0O₂-PEG-OH 5400 (1.25 g, 0.23 mmole) wasazeotropically distilled with 100 ml acetronitrile and then cooled toroom temperature. To this mixture was added disuccinimidyl carbonate(245 milligram, 0.92 mmole) and 0.1 ml of pyridine, and the solution wasstirred at room temperature overnight. The solvent was then removedunder vacuum, and the resulting solid was dissolved in 35 ml of drymethylene chloride. The insoluble solid was removed by filtration, andthe filtrate was washed with pH 4.5 sodium chloride saturated acetatebuffer. The organic phase was dried over anhydrous sodium sulfate,filtered, condensed by rotary evaporation, and precipitated into ethylether. The product was collected by filtration and dried in vacuo.

Yield: 1.20 g (96%), 100% substitution of succimidyl carbonate and noreagent left. NMR (DMSO-d₆): δ 3.5 (br m, PEG), 2.55 (t, —OCH₂CH₂COOPEG-), 4.14 (s, -PEG-OCH ₂COOPEG-), 4.13 (t, -PEGOCH₂CH₂COOCH₂CH₂OPEG-), 4.18 (t, -PEGOCH₂—COOCH ₂CH₂OPEG), 4.45 (t, -PEGO-CH₂CH₂OCONHS), 2.81 (s, NHS).

The invention has been described in particular exemplified embodiments.However, the foregoing description is not intended to limit theinvention to the exemplified embodiments, and the skilled artisan shouldrecognize that variations can be made within the scope and spirit of theinvention as described in the foregoing specification. On the contrary,the invention includes all alternatives, modifications, and equivalentsthat may be included within the true spirit and scope of the inventionas defined by the appended claims.

What is claimed is:
 1. A polymer having the following structure:POLY[—O—(CH₂)_(n′)—CO₂—(CH₂)_(m)—CO₂—NHS]_(n) wherein: POLY is afour-arm, branched PEG having a molecular weight from 300 to 100,000daltons; n′ is 1 to 5; m is 2 to 5; n is four; and NHS is succinimidyl.2. The polymer of claim 1, wherein the four-arm, branched PEG has apentaerythritol core.